Showing papers in "Journal of Mechanisms and Robotics in 2020"

Wrench-Feasible Workspace of Mobile Cable-Driven Parallel Robots

Abstract: Cable-Driven Parallel Robots hold numerous advantages over conventional parallel robots in terms of high speed and large workspace. Cable Drive Parallel Robots whose workspace can be further increased by the modification of their geometric architecture are known as Reconfigurable Cable Driven Parallel Robots. A novel concept of Reconfigurable Cable Driven Parallel Robots that consists of a classical Cable-Driven Parallel Robot mounted on multiple Mobile Bases is known as Mobile CDPR. This paper proposes a methodology to trace the Wrench-Feasible-Workspace of Mobile Cable-Driven Parallel Robots by determining its Available Wrench Set. Contrary to classical Cable-Driven Parallel Robots, we show that the Available Wrench Set of a Mobile Cable-Driven Parallel Robot depends, not only on the cable tension limits, but also on the Static Equilibrium conditions of the Mobile Bases. The Available Wrench Set is constructed by two different approaches known as Convex Hull approach and Hyperplane Shifting Method. Three case studies are carried out for the validation of the proposed methodology. The proposed approach is experimentally validated on a Mobile Cable-Driven Parallel Robot with a point-mass end-effector and two Mobile Bases.

An Origami-Based Medical Support System to Mitigate Flexible Shaft Buckling

Abstract: This paper presents the development of an origami-inspired support system (the OriGuide) that enables the insertion of flexible instruments using medical robots. Varying parameters of a triangulated cylindrical origami pattern were combined to create an effective highly compressible anti-buckling system that maintains a constant inner diameter for supporting an instrument and a constant outer diameter throughout actuation. The proposed origami pattern is composed of two repeated patterns: a bistable pattern to create support points to mitigate flexible shaft buckling and a monostable pattern to enable axial extension and compression of the support system. The origami-based portion of the device is combined with two rigid mounts for interfacing with the medical robot. The origami-based portion of the device is fabricated from a single sheet of polyethylene terephthalate. The length, outer diameter, and inner diameter that emerge from the fold pattern can be customized to accommodate various robot designs and flexible instrument geometries without increasing the part count. The support system also adds protection to the instrument from external contamination.

Supernumerary Robotic Limbs to Assist Human Walking With Load Carriage

Abstract: Walking with load carriage is a common requirement for individuals in many situations. Legged exoskeletons can transfer the load weight to the ground with rigid-leg structures, thus reducing the load weight borne by the human user. However, the inertia of paralleled structures and the mechanical joint tend to disturb natural motions of human limbs, leading to high-energy consumption. Different from exoskeletons, Supernumerary Robotic Limbs (SuperLimbs) are kinematically independent of the human limbs, thus avoiding the physical interference with the human limbs. In this paper, a SuperLimb system is proposed to assist the human walking with load carriage. The system has two rigid robotic limbs, and each robotic limb has four degrees-of-freedom (DOFs). The SuperLimbs can transfer the load weight to the ground through the rigid structures, thus reducing the weight borne by the human user. A hybrid control strategy is presented to assist the human as well as avoid disturbing user’s natural motions. Motions of the SuperLimb system are generated autonomously to follow the gait of the human user. The gait synchronization is controlled by a finite state machine, which uses inertial sensors to detect the human gait. Human walking experiments are conducted to verify this concept. Experiments indicate that the SuperLimbs can follow the human gait as well as distribute the load weight. Results show that our SuperLimb system can reduce 85.7% of load weight borne by the human when both robotic limbs support and 55.8% load weight on average. This study may inspire the design of other wearable robots and may provide efficient solutions for human loaded walking.

A Lower Limb Exoskeleton Recycling Energy From Knee and Ankle Joints to Assist Push-Off

Abstract: This paper presents the design and preliminary evaluation of a quasi-passive lower limb exoskeleton for walking efficiency improvements. The exoskeleton recycles the negative work performed by the knee joint in late swing phase and the ankle joint in mid-stance phase, to assist ankle push-off in late-stance phase when a burst of positive power is needed. The exoskeleton consists of a torsion spring as an energy storage element, and two clutches attached to both ends of the spring to control the timing of recycling and releasing energy in a gait cycle. The two clutches are actively controlled by two small servo motors with very low power consumption based on the plantar pressure. The novelty of this exoskeleton is it makes the extra kinetic energy dissipated at the knee joint reusable, by transferring it to the ankle joint to assist positive power generation during push-off, for the first time. Eight male subjects walked with the exoskeleton engaged (EXO_ON), disengaged (EXO_OFF), and without the exoskeleton (NO_EXO). Inverse dynamics analysis demonstrated reduced negative biological work at the knee joint during late swing and at the ankle joint during mid stance, as well as reduced positive biological work at the ankle joint during late stance comparing the EXO_ON to EXO_OFF conditions. These results prove the effectiveness of the exoskeleton at joint level.

A Mechanically Intelligent Crawling Robot Driven by Shape Memory Alloy and Compliant Bistable Mechanism

Abstract: Mechanical components in a robotic system were used to provide body structure and mechanism to achieve physical motions following the commands from electronic controller. This kind of robotic system utilizes complex hardware and firmware for sensing and planning. To reduce computational cost and increase reliability for a robotic system, employing mechanical components to fully or partially take over control tasks is a promising way, which is also referred to as “mechanical intelligence” (MI). This paper proposes a shape memory alloy driven robot capable of using a reciprocating motion to crawl over a surface without any use of electronic controller. A mechanical logic switch is designed to determine the activation timing for a pair of antagonistic shape memory alloy (SMA) actuators. Meanwhile, a compliant pre-strain bistable mechanism is introduced to cooperate with the SMA actuators achieving reliable reciprocating motion between the two stable positions. The SMA actuator is modeled base on a static two-state theory while the bistable mechanism is described by combining a pseudo-rigid-body model (PRBM) with a Bi-beam constraint model (Bi-BCM). Following this, the design parameters of the bistable mechanism and SMA actuators are determined according to theoretical simulations. Finally, a robotic prototype is fabricated using anisotropic friction on its feet to convert the reciprocating motion of the actuator to uni-directional locomotion of the robot body over a surface. Experiments are carried out to validate the presented design concept and the modeling methods.

Mechanical Design and Performance Analysis of a Novel Parallel Robot for Ankle Rehabilitation

Abstract: As the population ages, increasingly more individuals experience ankle disabilities caused by stroke and cerebral palsy. Studies on parallel robots for ankle rehabilitation have been conducted under this circumstance. This paper presents a novel parallel ankle rehabilitation robot with the key features of a simple configuration and actuator nonredundancy. The mechanical design is determined, and a prototype is built. Additionally, inverse position solution is addressed to calculate the workspace of the parallel robot. Jacobian matrices mapping the velocity and force from the active joint space to the task space are derived, and kinetostatic performance indices, namely, motion isotropy, force transfer ratio, and force isotropic radius are defined. Moreover, the inverse dynamic model is presented using the Newton–Euler formulation. Dynamic evaluation index, i.e., dynamic uniformity, is proposed according to the derived Jacobian matrix and inertia matrix. Based on the workspace analysis, the parallel robot demonstrates a sufficient workspace for ankle rehabilitation compared with measured range of motion of human ankle joint complex. The results of the kinetostatic and dynamic performance analysis indicate that the parallel robot possesses good motion isotropy, high force transfer ratio, large force isotropic radius, and relatively uniform dynamic dexterity within most of the workspace, especially in the central part. A numerical example is presented to simulate the rehabilitation process and verify the correctness of the inverse dynamic model. The simplicity and the performance of the proposed robot indicate that it has the potential to be widely used for ankle rehabilitation.

Large-Deflection Analysis of General Beams in Contact-Aided Compliant Mechanisms Using Chained Pseudo-Rigid-Body Model

Abstract: The nonlinear analysis and design of contact-aided compliant mechanisms (CCMs) are challenging. This paper presents a nonlinear method for analyzing the deformation of general beams that contact rigid surfaces in CCMs. The large deflection of the general beam is modeled by using the chained pseudo-rigid-body model. A geometry constraint from the contact surface is developed to constrain the beam’s deformed configuration. The contact analysis problem is formulated based on the principle of minimum potential energy and solved using an optimization algorithm. Besides, a novel technique based on the principle of work and energy is proposed to calculate the reaction force/moment of displacement-loaded cases. Several analysis examples of the compliant mechanisms with straight or curved beams are used to verify the proposed method. The results show that the proposed method and technique can evaluate the deformation of beam-based CCMs and the reaction force/moment with acceptable accuracy, respectively.

Origami-Layer-Jamming Deployable Surgical Retractor With Variable Stiffness and Tactile Sensing

Abstract: Origami-based flexible, compliant, and bio-inspired robots are believed to permit a range of medical applications within confined environments. In this article, we experimentally demonstrated an origami-inspired deployable surgical retractor with the controllable stiffness mechanism that can facilitate safer instrument–tissue interaction in comparison to their rigid counterparts. When controllable negative-pressure is applied to the jammed origami retractor module, it becomes more rigid, increasing its strength. To quantify origami-modules strength further, we demonstrated performances of retractor based on the Daler–Rowney Canford paper (38 grams per square meter (gsm)) and sandpaper of 1000 grit. Experiments on the proposed retractor prototype elucidated sandpaper-based retractor can outperform paper-38-gsm retractor for facelift incision with the width of more than 9 cm. Though 38 gsm Canford paper comprised of thin layers, 16 times lesser in thickness than sandpaper, experiments proved its comparable layer jamming (LJ) performance. We leverage the advantage of the LJ mechanism to tune retractor stiffness, allowing the instrument to hold and separate a facelift incision to mitigate the likelihood of surgical complications. The retractor is equipped with a custom-made printed conductive ink-based fabric piezoresistive tactile sensor to assist clinicians with tissue-retractor interaction force information. The proposed sensor showed a linear relationship with the applied force and has a sensitivity of 0.833 N−1. Finally, cadaver experiments exhibit an effective origami-inspired surgical retractor for assisting surgeons and clinicians in the near future.

A Bar and Hinge Model for Simulating Bistability in Origami Structures With Compliant Creases

Abstract: Active origami structures usually have creases made with soft and compliant plates because it is difficult to fabricate real hinges and actuate them. However, most conventional origami modeling techniques fail to capture these compliant creases and simplify them as concentrated rotational springs, which neglects torsional and extensional deformations of the creases. In this paper, an improved formulation of a bar and hinge model is proposed to explicitly capture the geometry and the flexibility of compliant creases with nonnegligible width in an origami, and the model is verified against finite element simulations. The verification shows that the model performs relatively well despite being simple and computationally inexpensive. Moreover, simulation examples demonstrate that the proposed model can capture the bistable behavior of the compliant crease origami with nonnegligible crease width because it explicitly includes the extensional stretching energy into the simulation framework and allows torsional crease deformations.

Torsional Stiffness Improvement of a Soft Pneumatic Finger Using Embedded Skeleton

Abstract: Silicone-based pneumatic actuators are among the most widely used soft actuators in adaptable fingers. However, due to the soft nature of silicone, the performance of these fingers is highly affected by the low torsional stiffness, which may cause failure in grasping and manipulation. To address this problem, a compact design is proposed by embedding a rigid skeleton into a soft pneumatic finger. A finite element approach with an analysical model is used to evaluate the performance of the fingers both with and without the skeleton. Then, a series of experiments is performed to study the bending motion and rigidity of the fingers. The results reveal that the skeleton increases the torsional stiffness of the finger up to 300%. Furthermore, the consistency with the experimental data indicates the good precision of the proposed modeling method. Finally, a two-finger hand is designed to evaluate the performance of the reinforced finger in reality. The grasp experiments illustrate that the hybrid finger with the skeleton is highly adaptable and can successfully grasp and manipulate heavy objects. Thus, a potential approach is proposed to improve the torsional stiffness of silicone-based pneumatic fingers while maintaining adaptability.

Screw Theory-Based Motion/Force Transmissibility Analysis of High-Speed Parallel Robots With Articulated Platforms

Abstract: Motion/force transmissibility is an essential property reflecting the kinematic performance of parallel robots. Research on this performance of the single-platform parallel robots (SPPRs) has long been concerned and studied. In contrast, although many innovations and applications of the high-speed articulated-platform parallel robots (APPRs) have been presented, few studies on their motion/force transmissibility have been reported. This paper deals with the motion/force transmissibility analysis of high-speed parallel robots with articulated platforms. A modified output transmission index (MOTI) for the high-speed parallel robots with articulated platforms is proposed based on a newly defined concept of equivalent transmission wrench screw. Furthermore, by having an insight into the instantaneous relative motion inside the mobile platform, a medial transmission index (MTI) is proposed to evaluate its internal motion/force transmissibility. Based on these foundations, the local transmission index (LTI) is redefined as the minimum value of the input, modified output, and medial transmission indices. Under the framework of the above performance indices, motion/force transmissibility analysis of two typical high-speed articulated-platform parallel robots, i.e., Heli4 and Par4, are presented. The proposed indices are excepted to be applied to the optimal design of high-speed parallel robots with articulated platforms.

Gravity Compensation Design of Planar Articulated Robotic Arms Using the Gear-Spring Modules

Abstract: This paper presents a design concept for gravity compensation of planar articulated robotic arms using a series of gear-slider mechanisms with springs. The spring-attached gear-slider mechanism has one degree-of-freedom (DOF) of motion, which can serve as a gear-spring module (GSM) to be installed onto the robot joints for leveraging the gravitational energy of the robot arm. The proposing GSM-based design is featured by its structure compactness, less assemblage effort, ease of modularization, and high performance for gravity compensation of articulated robotic manipulators. As a key part of the design, the stiffness of the spring in the GSM can be determined through either a design optimization or an analytical approximation to perfect balancing. The analyses on several 1-, 2-, and 3-DOF GSM-based robot arms illustrate that the analytical approximation to perfect balancing can reach nearly the same performance as provided through the design optimization. The power loss due to the gear contact is considered when evaluating the gravity compensation performance. A formula for spring stiffness correction is suggested for taking the power loss into account. An experimental study on a one-DOF GSM-based robot arm was performed, which shows that a power reduction rate of 86.5% is attained by the actuation motor when the GSM is installed on the robot arm.

A Novel Style Design of a Permanent-Magnetic Adsorption Mechanism for a Wall-Climbing Robot

Abstract: Magnetic adsorption mechanisms are widely used for wall-climbing robots to manipulate a locomotive on the surface of a magnetic conducting metal. However, the reported magnetic adsorption mechanisms are subject to the problems such as the lack of adsorption capability, the weakness of kinematic performance, and the overwhelming detaching force. To solve the problems, a novel style of a permanent-magnetic adsorption mechanism using an electromagnetic method and internal force compensation principle is detailed in this work. Specifically, a permanent magnet, an electromagnet, and a nonlinear spring are configurated to achieve a reliable adsorption function by using the minimal detaching force. Following that, the results obtained from both the finite element analysis and the experiments carried out by using a prototype demonstrated its effectiveness. It does not only have a rapid and controllable adsorption-detachment capacity in reference to the magnetic conducting surface but also has low power consumption, large adsorption force, and reliable and safe performance.

A Fully Compliant Tristable Mechanism Employing Both Tensural and Compresural Segments

Abstract: A multistable compliant mechanism is a device that can hold several distinct positions through the storage and release of the strain energy associated with deflections of the flexible members. This self-locking capability can benefit many applications such as threshold acceleration sensing, overload protection, and shape reconfiguration. This work presents a novel class of fully compliant tristable mechanisms called tensural–compresural tristable mechanisms (TCTMs), which forms three stable equilibrium positions through unique utilization of both tensural segments and compresural segments. To identify feasible designs, a kinetostatic model is developed using the chained beam-constraint-model (CBCM) for both tensural segments and compresural segments. Two TCTM designs accompanied with a prototype are presented to demonstrate the feasibility of this new tristable configuration and the effectiveness of the kinetostatic model.

A Translational Three-Degrees-of-Freedom Parallel Mechanism With Partial Motion Decoupling and Analytic Direct Kinematics

Abstract: According to the topological design theory and method of parallel mechanism (PM) based on position and orientation characteristic (POC) equations, this paper studied a 3-DOF translational PM that has three advantages, i.e., (i) it consists of three fixed actuated prismatic joints, (ii) the PM has analytic solutions to the direct and inverse kinematic problems, and (iii) the PM is of partial motion decoupling property. Firstly, the main topological characteristics, such as the POC, degree of freedom and coupling degree were calculated for kinematic modeling. Thanks to these properties, the direct and inverse kinematic problems can be readily solved. Further, the conditions of the singular configurations of the PM were analyzed which corresponds to its partial motion decoupling property.

Analytical Study of the Snap-Through and Bistability of Beams With Arbitrarily Initial Shape

Abstract: We derive the snap-through solution and the governing snapping force equations for an arbitrarily pre-shaped beam deflected under a mid-length lateral point force. The exact solution is obtained based on the classical theory of elastic beams as a superposition of the initial shape and the modes of buckling. Two kinds of solution are identified depending on the axial force level. The two solutions, bifurcation conditions, bistability conditions, and the snapping force equations are derived and discussed. The snap-through and snapping force solutions are then calculated for two common beam initial shapes, the curved (first buckling shape) and the inclined one (V-shape). In both cases, explicit expressions are obtained describing the snap-through behavior. The analytical modeling results show excellent agreement with the finite element simulations. The comparison between the two cases shows a similar snap-through behavior qualitatively, while several differences and similarities are noticed quantitatively.

Optimization of the Rotational Asymmetric Parallel Mechanism for Hip Rehabilitation With Force Transmission Factors

Abstract: An asymmetric three-degree-of-freedom parallel mechanism is adopted in rehabilitation robots for assisting patients suffering from stroke or trauma in the hip. It is necessary to keep its kinematic singularity out of the workspace of human normal gait and increase the output power efficiency. Therefore, a novel method is proposed to optimize geometrical parameters of the mechanism. To describe the kinematic singularity in a better way, the improved force transmission indexes based on previous methods are proposed using the reciprocal product and mobility condition of the closed-loop mechanism. The indexes mainly represent the force transmission performance of unactuated parts of subchains and moving platform. Together with the driving force transmission indexes and geometrical constraints, the multiobjective optimization model is established. The differential evolution algorithm, which is widely applied to mechanism optimization, is used to achieve optimal results. The Jacobian matrix singularity and output power efficiency along giving trajectory before and after optimization are compared to verify the effectiveness of the method.

On the Optimal Resolution of Inverse Kinematics for Redundant Manipulators Using a Topological Analysis

Abstract: The redundancy resolution schemes based on the optimization of an integral performance index are investigated from the topological point of view. The topological notions of self-motion manifold, C-path-homotopy and extended aspect are clarified in relation to the limitations of the necessary conditions of optimality provided by calculus of variations. On one hand, they do not guarantee the achievement of the optimal solution, and on the other hand, they translate into a two-point boundary value problem (TPBVP), whose resolution, under certain circumstances, may not lead to a feasible solution at all. In response to the limitations of calculus of variations, a dynamic-programming-inspired formalism is developed, which is based on the discretization of the state space and on its representation in the form of multiple grids. Building upon the topological analysis, effective algorithms are designed that are able to find the optimal solution in any condition, across all C-path homotopy classes and self-motion manifolds, with no limitation due to the passage through singularities. Moreover, if the grids are representative of the manipulator’s extended aspects, the topological notion of the transitional point can be used to reduce the computational complexity of the optimal redundancy resolution algorithm. The results are demonstrated on a canonical 4R planar robot in two different scenarios.

Design, Control, and Pilot Study of a Lightweight and Modular Robotic Exoskeleton for Walking Assistance After Spinal Cord Injury

Abstract: Walking rehabilitation using exoskeletons is of high importance to maximize independence and improve the general well-being of spinal cord injured subjects. We present the design and control of a lightweight and modular robotic exoskeleton to assist walking in spinal cord injured subjects who can control hip flexion, but lack control of knee and ankle muscles. The developed prototype consists of two robotic orthoses, which are powered by a motor-harmonic drive actuation system that controls knee flexion–extension. This actuation module is assembled on standard passive orthoses. Regarding the control, the stance-to-swing transition is detected using two inertial measurement units mounted on the tibial supports, and then the corresponding motor performs a predefined flexion–extension cycle that is personalized to the specific patient’s motor function. The system is portable by means of a backpack that contains an embedded computer board, the motor drivers, and the battery. A preliminary biomechanical evaluation of the gait-assistive device used by a female patient with incomplete spinal cord injury at T11 is presented. Results show an increase of gait speed (+24.11%), stride length (+7.41%), and cadence (+15.56%) when wearing the robotic orthoses compared with the case with passive orthoses. Conversely, a decrease of lateral displacement of the center of mass (-19.31%) and step width (-13.37% right step, -8.81% left step) are also observed, indicating gain of balance. The biomechanical assessment also reports an overall increase of gait symmetry when wearing the developed assistive device.

Mobility Analysis of the Threefold-Symmetric Bricard Linkage and Its Network

Abstract: In this study, the mobility of the threefold-symmetric Bricard linkage and its network are analyzed using screw theory. First, the screw motion equation of the linkage is derived. By applying the modified Grübler–Kutzbach criterion, we deduce that the degree of freedom (DOF) of the linkage is equal to 1. Then, we analyze the mechanical network constructed of threefold-symmetric Bricard linkages and provide its topological constraint graph. Using graph theory and screw theory, the constraint matrix of the mechanical network is obtained. Then, we solve the matrix rank via linear column transformation. Results show that the DOF of the mechanical network is equal to 1. The mobility analysis method of the mechanical network proposed in this study facilitates the solution of the constraint matrix rank and can be used as a reference for other mechanical networks constructed of single-loop linkages.

Degree of Freedom and Dynamic Analysis of the Multi-Loop Coupled Passive-Input Overconstrained Deployable Tetrahedral Mechanisms for Truss Antennas

Abstract: Recently, the truss antennas with deployable tetrahedron unit mechanisms have been successfully applied in orbit, owing to the advantages of large calibers, high accuracy, and large folding ratios. As multiloop coupled mechanisms, deployable tetrahedral mechanisms have multiple different output links, whose supporting limbs connecting output links and the base are mutually coupled. These mechanisms are also called the passive-input overconstrained mechanisms because their passive torsion springs are used as drivers and because the number of the drivers contained is more than the degrees of freedom (DOFs). In this work, a method based on the equivalent concept of first link-removing and then restoring is proposed for the DOF analysis of the multiloop coupled deployable tetrahedral mechanisms. With one coupled chain removed, the equivalent serial chains between the coupled components and the base are established in the remainder of the mechanisms. Then, the coupled chain removed is restored and the equivalent of the multiloop coupled mechanisms is obtained. The Lagrange method is used to establish the dynamic equation of the passive-input overconstrained mechanisms; the influence of the stiffness and number of torsion springs on the unfolding motion is examined.

A Lumped-Mass Model for Large Deformation Continuum Surfaces Actuated by Continuum Robotic Arms

Abstract: Currently, flexible surfaces enabled to be actuated by robotic arms are experiencing high interest and demand for robotic applications in various areas such as healthcare, automotive, aerospace, and manufacturing. However, their design and control thus far has largely been based on “trial and error” methods requiring multiple trials and/or high levels of user specialization. Robust methods to realize flexible surfaces with the ability to deform into large curvatures therefore require a reliable, validated model that takes into account many physical and mechanical properties including elasticity, material characteristics, gravity, external forces, and thickness shear effects. The derivation of such a model would then enable the further development of predictive-based control methods for flexible robotic surfaces. This paper presents a lumped-mass model for flexible surfaces undergoing large deformation due to actuation by continuum robotic arms. The resulting model includes mechanical and physical properties for both the surface and actuation elements to predict deformation in multiple curvature directions and actuation configurations. The model is validated against an experimental system where measured displacements between the experimental and modeling results showed considerable agreement with a mean error magnitude of about 1% of the length of the surface at the final deformed shapes.

Reconfiguration of a 3-(rR)PS Metamorphic Parallel Mechanism Based on Complete Workspace and Operation Mode Analysis

Abstract: This paper focuses on the reconfiguration of a 3-(rR)PS metamorphic parallel mechanism based on complete workspace and operation mode analysis. The mechanism consists of three (rR)PS legs, and each (rR) joint is composed of two perpendicular revolute joints. One of the (rR) joint axes can be reconfigured continuously, which allows the mechanism to exhibit three distinct configurations. Initially, the constraint equations are derived by using algebraic geometry approach, and the primary decomposition is computed for the three configurations. It reveals that the 3-(rR)PS metamorphic parallel mechanism can exhibit one up to two operation modes among three configurations. When the second axes of the three (rR) joints intersect at a finite point and not coplanar, the 3-(rR)PS metamorphic parallel mechanism has only one operation mode. If the second axes of the three (rR) joints are coplanar, the 3-(rR)PS metamorphic parallel mechanism has two operation modes. It is shown that both operation modes have the same motion type, namely, 1T2R motion. However, to realize the same trajectories in both operation modes, the moving platform will have different orientations. Hence, the orientation workspaces of both operation modes are characterized and the axodes are used to compare the instantaneous motion of the moving platform when passing through the same trajectories. Based on these results, an identification approach is introduced to identify which operation mode a given mechanism pose belongs to and this provides a useful method for trajectory planning.

Forward Kinematic Analysis of Kinematically Redundant Hybrid Parallel Robots

Abstract: This paper focuses on the forward kinematic analysis of (6 + 3)-degree-of-freedom kinematically redundant hybrid parallel robots. Because all of the singularities are avoidable, the robot can cover a very large orientational workspace. The control of the robot requires the solution of the direct kinematic problem using the actuator encoder data as inputs. Seven different approaches of solving the forward kinematic problem based on different numbers of extra encoders are developed. It is revealed that five of these methods can produce a unique solution analytically or numerically. An example is given to validate the feasibility of these approaches. One of the provided approaches is applied to the real-time control of a prototype of the robot. It is also revealed that the proposed approaches can be applied to other kinematically redundant hybrid parallel robots proposed by the authors.

A Monolithic Compliant Continuum Manipulator: A Proof-of-Concept Study

Abstract: Continuum robots have the potential to form an effective interface between the patient and surgeon in minimally invasive procedures. Magnetic actuation has the potential for accurate catheter steering, reducing tissue trauma and decreasing radiation exposure. In this paper, a new design of a monolithic metallic compliant continuum manipulator is presented, with flexures for precise motion. Contactless actuation is achieved using time-varying magnetic fields generated by an array of electromagnetic coils. The motion of the manipulator under magnetic actuation for planar deflection is studied. The mean errors of the theoretical model compared to experiments over three designs are found to be 1.9 mm and 5.1degrees in estimating the in-plane position and orientation of the tip of the manipulator, respectively and 1.2 mm for the whole shape of the manipulator. Maneuverability of the manipulator is demonstrated by steering it along a path of known curvature and also through a gelatin phantom which is visualized in real time using ultrasound imaging, substantiating its application as a steerable surgical manipulator.

Design, Analysis, and Integration of a New Two-Degree-of-Freedom Articulated Multi-Link Robotic Tail Mechanism

Abstract: Based on observations from nature, tails are believed to help animals achieve highly agile motions. Traditional single-link robotic tails serve as a good simplification for both modeling and implementation purposes. However, this approach cannot explain the complicated tail behaviors exhibited in nature where multi-link structures are more commonly observed. Unlike its single-link counterpart, articulated multi-link tails essentially belong to the serial manipulator family which possesses special motion transmission design challenges. To address this challenge, a cable-driven hyper-redundant design becomes the most used approach. Limited by cable strength and elastic components, this approach suffers from low-frequency response, inadequate generated inertial loading, and fragile hardware, which are all critical drawbacks for robotic tails design. To solve these structure-related shortcomings, a multi-link robotic tail made up of rigid links is proposed in this paper. The new structure takes advantage of the traditional hybrid mechanism architecture, but utilizes rigid mechanisms to couple the motions between the ith link and the (i + 1)th link rather than using cable actuation. By doing so, the overall tail becomes a rigid mechanism that achieves quasi-uniform spatial bending for each segment and allows performing highly dynamic motions. The mechanism and detailed design of this new robotic tail are presented. The kinematic model was developed and an optimization process was conducted to reduce the bending non-uniformity for the rigid tail. Based on this special optimization design, the dynamic model of the new mechanism is significantly simplified. A small-scale three-segment prototype was integrated to verify the proposed mechanism's unique mobility.

Discrete Layer Jamming for Variable Stiffness Co-Robot Arms

Abstract: Continuous layer jamming is an effective tunable stiffness mechanism that utilizes vacuum to vary friction between laminates enclosed in a membrane. In this paper, we present a discrete layer jamming mechanism that is composed of a multilayered beam and multiple variable pressure clamps placed discretely along the beam; system stiffness can be varied by changing the pressure applied by the clamps. In comparison to continuous layer jamming, discrete layer jamming is simpler as it can be implemented with dynamic variable pressure actuators for faster control, better portability, and no sealing issues due to no need for an air supply. Design and experiments show that discrete layer jamming can be used for a variable stiffness co-robot arm. The concept is validated by quasi-static cantilever bending experiments. The measurements show that clamping 10% of the beam area with two clamps increases the bending stiffness by around 17 times when increasing the clamping pressure from 0 to 3 MPa. Computational case studies using finite element analysis for the five key parameters are presented, including clamp location, clamp width, number of laminates, friction coefficient, and number of clamps. Clamp location, number of clamps, and number of laminates are found to be most useful for optimizing a discrete layer jamming design. Actuation requirements for a variable pressure clamp are presented based on results from laminate beam compression tests.

Sensorless In-Hand Manipulation by An Underactuated Robot Hand

Abstract: The limited dexterity that existing hand prostheses provide to users contrasts with the manipulation abilities exhibited by state-of-the-art robot hands. This paper presents an underactuated robot hand with in-hand manipulation capabilities to demonstrate the use of underactuation in the development of effective hand replacements. This paper describes a specific underactuated hand architecture, representative of many existing underactuated hand prototypes. First, the hand is modeled and its ability to manipulate objects of different geometries is analyzed. Second, a manipulation strategy suitable for prosthetic applications is proposed. The strategy enables the hand to manipulate objects in-hand without any a priori information of their geometry or physical properties. Finally, experimental tests conducted to validate the theoretical results are presented.

Workspace Analysis and Optimal Design of a Translational Cable-Driven Parallel Robot With Passive Springs

Abstract: Cable-driven parallel robots (CDPRs) have great prospects for high-speed applications because of their nature of low inertia and good dynamics. Existing high-speed CDPRs mainly adopt redundant cables to keep positive cable tensions. Redundant cables lead to complex and costly structure, and are likely to cause interference. In this study, a non-redundant CDPR for high-speed translational motions is designed with passive springs and parallel cables. First, the configuration of the CDPR is illustrated, and its kinematics and dynamics are studied. Then, the workspace of the CDPR is discussed in detail. The condition of positive cable tensions is proved. The influence of the springs’ layout on the workspace is analyzed. A method for determining the regular cylindrical operation workspace is proposed. Furthermore, the optimal design method for high-speed CDPRs with passive springs is developed. Performance indices for evaluating the force transmission are defined based on the matrix orthogonal degree. The geometric parameters are optimized based on the workspace and force transmission indices. The stiffness coefficient of the spring is determined based on the acceleration and cable tension requirements. Finally, the proposed CDPR and the traditional CDPR with redundant cables are compared through simulation. The results show that the designed CDPR possesses advantages in energy consumption and simple structure compared to CDPR with redundant cables.

Modeling and Simulation of Robotic Finger Powered by Nylon Artificial Muscles

Abstract: A robotic finger actuated by novel artificial muscles known as twisted and coiled polymer (TCP) muscles has been proposed as an inexpensive, yet high-performance component of a robotic hand in recent years. In this paper, the Euler–Lagrangian method coupled with an electro-thermo-mechanical model-based transfer function was used for the analysis of finger joints in the hand. Experiments were performed at three power magnitudes provided to the TCP muscles, and the output angular displacements of the index finger subtended corresponding to the power levels were measured. The measured input and output parameters were used for system identification. To elucidate how the new artificial muscle influences the finger motion, two types of numerical simulations were performed: force input simulation (FIS) using measured force as an input and power input simulation (PIS) using measured electrical power as an input. Results were quantified statistically, and the simulated data were compared with the experimental results. Sensitivity analysis was also presented to understand the effect of the mechanical properties on the system. This model will help in understanding the effect of the TCP muscles and other similar smart actuators on the dynamics of the robotic finger.